Alkylene carbonate is used for various purposes, such as an electrolytic element of lithium secondary batteries, a polar aprotic solvent, a monomer for synthesizing polycarbonate or other polymer, a chemical ingredient for preparing medicines or agricultural chemicals, and an alkylating agent.
Heretofore, alkylene carbonate has been produced by reacting alkylene oxide with carbon dioxide under normal or higher pressure and in the presence of a homogeneous catalyst such as alkali metal halide, ammonium salt or phosphonium salt.
In the method using the homogeneous catalyst, the catalyst is undesirably dissolved in a phase containing a reaction product or alkylene carbonate. Thus, it is required to separate the catalyst from the product through a process such as distillation after completion of the reaction, resulting in complicated production processes, and decomposition of the catalyst or formation of by-products during the separation process.
In order to facilitate the separation of the catalyst, there have been proposed various solid catalysts, such as an ion exchange resin containing a quaternary ammonium group or a quaternary phosphonium group as an exchange group (Japanese Patent Publication Nos. 3-120270, 7-206846, 7-206848, 9-235252 etc.), a heteropoly acid or salt thereof (Japanese Patent Publication No. 7-206847), MgO (T. Yano et al., Chem. Commun., 1129 (1997)), and a complex oxide obtained by burning a basic layered compound (Japanese Patent Publication No. 11-226413; K. Yamaguchi et al., J. Am. Chem. Soc., 121, 4526 (1999)).
However, these solid catalysts have insufficient activity because their active site is limited to a solid surface, due to its insolubility to a reaction product and reaction components, such as carbon dioxide and alkylene oxide, and, in addition, they are subject to interference from dissolved reaction components. While the reaction temperature can be increased to improve activity, it may deteriorate catalyst selectivity. In particular, the ion exchange resin catalysts have a limited reaction temperature due to poor heat-resistance. Most solid catalysts are essentially required to contain an aprotic polar solvent as an additive for assuring activity and selectivity. In addition, most solid catalysts suffer from degradation due to elution of catalyst components into the reaction product or alkylene carbonate.